1. Field of the Invention
The invention relates to fluidized catalytic cracking and heat exchangers.
2. Description of Related Art
In the fluidized catalytic cracking (FCC) process, catalyst, having a particle size and color resembling table salt and pepper, circulates between a cracking reactor and a catalyst regenerator. In the reactor, hydrocarbon feed contacts a source of hot, regenerated catalyst. The hot catalyst vaporizes and cracks the feed at 425.degree. C.-600.degree. C., usually 460.degree. C.-560.degree. C. The cracking reaction deposits carbonaceous hydrocarbons or coke on the catalyst, thereby deactivating the catalyst. The cracked products are separated from the coked catalyst. The coked catalyst is stripped of volatiles, usually with steam, in a catalyst stripper and the stripped catalyst is then regenerated. The catalyst regenerator burns coke from the catalyst with oxygen containing gas, usually air. Decoking restores catalyst activity and simultaneously heats the catalyst to, e.g., 500.degree. C.-900.degree. C., usually 600.degree. C.-750.degree. C. This heated catalyst is recycled to the cracking reactor to crack more fresh feed. Flue gas formed by burning coke in the regenerator may be treated for removal of particulates and for conversion of carbon monoxide, after which the flue gas is normally discharged into the atmosphere.
Catalytic cracking has undergone progressive development since the 40s. The trend of development of the fluid catalytic cracking (FCC) process has been to all riser cracking and use of zeolite catalysts. A good overview of the importance of the FCC process, and its continuous advancement, is reported in Fluid Catalytic Cracking Report, Amos A. Avidan, Michael Edwards and Hartley Owen, as reported in the Jan. 8, 1990 edition of the Oil & Gas Journal.
Modern catalytic cracking units use active zeolite catalyst to crack the heavy hydrocarbon feed to lighter, more valuable products. Instead of dense bed cracking, with a hydrocarbon residence time of 20-60 seconds, less contact time is needed. Conversion of feed can now be achieved in less time and more selectively in a dilute phase, riser reactor.
Although reactor residence time has continued to decrease, the height of the reactors has not decreased proportionally. The need for a somewhat vertical design, to accommodate the great height of the riser reactor, and the need to have a unit which is compact, efficient, and has a small "footprint" has caused considerable evolution in the design of FCC units, which evolution is reported to a limited extent in the Jan. 8, 1990 Oil & Gas Journal article. One modern, compact FCC design is the Kellogg Ultra Orthoflow converter, Model F, which is shown in FIG. 1 of this patent application, and also shown as FIG. 17 of the Jan. 8, 1990 Oil & Gas Journal article discussed above. The unit uses a regenerator consisting of a bubbling dense bed of catalyst. The regenerator has an external heat exchanger, which allows the unit to process heavy crudes, or those containing large amounts of Conradson Carbon Residue material, without overheating.
Such cooling of the catalyst regenerator is ancient art. In the 40's, many FCC regenerators had external catalyst coolers. With better unit design, and more active catalyst, the units ran "heat balanced" and did not require catalyst coolers. Today FCC units are being pushed to crack heavier and heavier feeds, which contain large amounts of CCR, so once again catalyst coolers are needed to permit heat balanced operation.
Coolers are now usually connected to the bubbling dense bed of catalyst in the regenerator, as in the Kellogg HOC design. These bubbling dense beds regenerate catalyst and store it until recycled to the riser reactor. Not all regenerators use bubbling dense bed as the primary means of removing carbon from spent catalyst.
The "High Efficiency Regenerator" (H.E.R.) design uses a fast fluidized bed for most of the coke combustion, and a dilute phase transport riser for some CO combustion. Regenerated catalyst is collected in a bubbling dense bed for reuse, and for recycle to the coke combustor. Such a design makes more efficient use of the catalyst, in that the coke combustor is all highly active, unlike bubbling dense bed regenerators, which are troubled with stagnant beds (due to poor catalyst flow patterns) and regeneration gas bypassing (due to the formation of large bubbles within the bubbling dense bed). The H.E.R. design uses a turbulent fluidized bed, or a fast fluidized bed, which allows use of less catalyst than is required in a bubbling dense bed regeneration design.
H.E.R. units have, like bubbling bed units, been pushed to deal with the increased carbon burning duties associated with cracking heavy crudes. The operation of these units has been modified to try to maintain heat balanced operation, either by limiting the heat release during regeneration (partial CO combustion mode) or by removing heat via heat exchange.
A partial combustion route to limiting heat release is disclosed in U.S. Pat. No. 4,849,091, which is incorporated herein by reference. Such an approach allows some of the heat release to be shifted to a downstream CO boiler. This takes some of the load from the regenerator, but is not an option at some refineries, or does not take enough heat from the regenerator. Thus many regenerators now run too hot, and some type of heat exchange is needed.
Adding heat exchangers to both bubbling bed and H.E.R. regenerators is reported in many patents.
In U.S. Pat. No. 4,439,533, incorporated by reference, a backmixed heat exchanger is added to the bubbling dense bed of catalyst. There are no slide valves, or elaborate catalyst supply and return lines to the heat exchanger, rather the heat exchanger is closely coupled, and in open fluid communication with the bubbling dense bed. The backmixed heat exchanger looks something like a thimble. Adjusting the amount of gas added to the "thimble" containing the heat exchange tubes allows some control of catalyst circulation and of heat exchange. Cooling fluid, usually water, passes through a tube bundle in the thimble. Cooled catalyst returns to the bubbling dense bed regenerator using the same opening as the entering catalyst.
In U.S. Pat. No. 4,434,245, incorporated by reference, a flow through heat exchanger is disclosed for use with a high efficiency regenerator. There is a hot catalyst inlet in the bubbling dense bed, and a cooled catalyst outlet in the coke combustor or FFB region. This approach requires a significant amount of hardware modifications, slide valves, and a fluidizing air outlet from the upper portion of the heat exchanger to the dilute phase region above the bubbling dense bed.
In U.S. Pat. No. 4,578,366 a flow through heat exchanger is used. Fluidizing gas in the heat exchanger supports combustion in the coke combustor. Catalyst slide valves regulate the flow of hot regenerated catalyst from the dense bed to the heat exchanger.
In U.S. Pat. No. 4,595,567 a flow through heat exchanger is used with heat pipes. Catalyst slide valves regulate the flow of hot regenerated catalyst from the bubbling dense bed into the heat exchanger.
In U.S. Pat. No. 4,430,302 a fast fluidized bed regenerator (without catalyst recycle) has looped heat exchange coils in the fast fluidized bed. Looped coils, formed from 11/2 or 2" 304H stainless steel were suspended in multiple banks in the bed. Such an approach will remove a lot of heat, but requires a lot of custom work and is hard to control. To avoid thermal shock refiners like to have heat exchange fluid flowing all the time through the tubes. Heat removal goes on all the time, even during startup when heat input rather heat removal is needed.
I reviewed this extensive art on coolers in regenerators but found nothing that was completely satisfactory. They either cost too much, or removed too little or too much heat, and/or were hard to control.
The flow through, dense phase, down flow exchanger is efficient in terms of heat removal and controllability, but requires separate catalyst inlets and returns, slide valves, and a lot of space. Capital costs are fairly high.
The dense phase back-mixed exchanger avoids some of the plumbing costs. It has only a single connection to the regenerator for solids entry and exit. This reduces the cost to install it, and reduces efficiency to about 60% of a flow through exchanger. The same opening is used for solids entry and exit, and the traffic jam restricts catalyst traffic.
I wanted to avoid use of slide valves to control catalyst flow to the regenerator. These can cost more than $1,000,000 each and are usually used in pairs to permit servicing.
I wanted to avoid coils within the regenerator. Cooling coils should always be full of coolant (to avoid thermal shock and damage to the tubes). Coils full of coolant remove heat even during startup, when the unit requires heating, not cooling. There is concern too that coils may interfere with fluidization within the regenerator.
I wanted a reliable and efficient way to remove heat from a regenerator during normal operation, which could be isolated during startup, while achieving efficient heat removal during normal operation. It was my goal to retain the mechanical simplicity and low cost of a backmixed, dense phase heat exchanger, but improve its efficiency to something approaching that of a flow through heat exchanger.
A baffling improvement provided the key to obtaining the performance of a flow through exchanger in a backmixed exchanger. Using a baffle also allowed frequent or periodic reversal of flow patterns through the regenerator, which can extend the life of the heat exchanger tubes. With proper design, I was also able to selectively cool either the catalyst which would be returned to the reactor, or the bulk of the catalyst in the regenerator.